ghostwriter vs playwright

ghostwriter

noun
  • A professional writer who is paid to write material that is officially credited to another person; one who writes on behalf of someone else, often for a celebrity. 

  • Smith was listed as the author of his autobiography, but a ghostwriter did most of the work. 

playwright

noun
  • A writer and creator of theatrical plays. 

How often have the words ghostwriter and playwright occurred in a corpus of books? (source: Google Ngram Viewer )